DESCRIPTION (Investigator's Abstract): The general aim of this project is to continue the investigation of protein metabolism in exercise and recovery in humans. Because of the investigator's previous results, one component of this proposal will study the response during exercise, and the other component will focus on the recovery period. The investigators will investigate the hypothesis that during exercise total urea production is maintained relatively constant because increased production from amino acids is balanced by a decreased production from recycled nitrogen (N) coming from the intestine. The investigators will use the primed-constant infusion of 13N2-urea to determine the rate of total urea production (from the enrichment of doubly-labeled urea) and the amount of urea produced from recycled urea N (from the enrichment of singly-labeled urea). The investigators will also investigate the hypothesis that the synthesis of intestinal mucosa protein is suppressed during exercise. The investigators propose that this is the net mechanism whereby amino acids are made available for the maintenance of muscle protein synthesis, despite accelerated amino acid oxidation. Studies of the response during recovery from exercise will focus on the mechanisms and nutritional implications of the stimulation of muscle protein synthesis they have observed previously. The investigators will continue to directly determine the rate of muscle protein synthesis by measuring the rate of incorporation of labeled leucine into muscle protein, obtained by biopsy. They will determine if following exercise there is an increased sensitivity to the stimulatory effect of insulin and/or amino acid infusion, or muscle protein synthesis. In addition, they will compare the response following resistance exercise with the response after aerobic training, to test the hypothesis that the response following resistance training is dependent on the muscle group exercised. These results will provide the fundamental knowledge necessary to formulate an optimal nutrition approach to the promotion of net protein synthesis following exercise.